The present invention relates to a coating liquid for forming a hydrophilic film on the surface of a substrate made of material such as glass, mirror, metal, or the like, and a method for producing such coating liquid.
Hitherto, for example, inorganic plate glass has been widely used for various articles such as window pane, mirror and eyeglass lenses, by the reason of its transparency and other good characteristics. When such article is used, for example, in a place of high temperature and high humidity, the dew condensation is caused on its surface, thereby to fog the same. In addition to this fogging problem, there has been another problem, for example, in the automotive outside mirror, to have many raindrops on its surface in the rainy weather. These problems interfere with the automotive rear view of the driver. Thus, there have been various proposals to provide the above-mentioned articles with antifogging property, water-drops preventive property, and durability. For example, there has been a proposal to form a hydrophilic film on the surface of a substrate such as glass plate, for the purpose of preventing fogging and the water-drops attachment thereto. There has been known for a long time a proposal of applying a surface active agent to the surface of such substrate, for achieving this purpose. For example, Japanese Patent Unexamined Publication JP-A-52-101680 discloses an antifogging agent for transparent article such as glass. This agent contains polyacrylic acid, a surface active agent, and a solvent which is water and/or alcohol. JP-A-55-154351 discloses a hydrophilic film formed on substrate. This film contains a phosphorus oxide and at least one of a molybdenum oxide and a tungsten oxide. JP-A-54-105120 discloses a method for producing an antifogging glass article by contacting an inorganic glass substrate containing P.sub.2 O.sub.5, with a P.sub.2 O.sub.5 --containing liquid or vapor. Furthermore, JP-A-53-58492 discloses an antifogging agent containing at least one sulfonic-acid-type amphoteric surface active agent represented by a general formula disclosed therein, and at least one particular inorganic salt or acetate represented by a general formula disclosed therein.
There are some proposals of forming a titania film on a substrate, for the purpose of decomposing contaminants or odorants contained in water or air by the photocatalytic activity of the titania film. For example, JP-A-63-97234 discloses a photocatalyst made of titanium oxide and an additive which is at least one element selected from the group consisting of platinum, palladium, and rhodium. JP-A-2-223909 discloses a multilayer coating film formed on eye glasses. This coating film is made up of a first layer of SiO.sub.2, a second color layer, and a third layer of TiO.sub.2, and each of these layers is formed by physical vapor deposition (PVD). JP-A-7-100378 discloses a photocatalytic titanium oxide thin film. This film is prepared by a method comprising the sequential steps of (a) mixing a titanium alkoxide with an alcohol amine, to prepare a titania sol; (b) applying the titania sol to a substrate; and (c) baking the resultant titania sol film by increasing its temperature from room temperature to a final temperature of from 600 to 700.degree. C. The resultant titania film is of the anatase type. After such titania film is irradiated with ultraviolet rays, contaminants attached to the titania film are decomposed by the photocatalytic activity of the titania film. With this, the titania film becomes hydrophilic. However, if such a conventional titania film is not irradiated with light for a long time, organic contaminants accumulate gradually on the titania film. With this, the titania film becomes inferior in hydrophilicity.